1. Field of the Invention
The present invention relates to a surface acoustic wave device for use in, for example, a band-pass filter or a resonator, and the present invention also relates to a method of producing such a surface acoustic wave device.
2. Description of the Related Art
Conventionally, surface acoustic wave devices have been widely used as band-pass filters and resonators. For application of surface acoustic wave devices as band-pass filters, resonators, or other such devices, the most important requirement is that the devices must have good frequency characteristics.
Generally, appropriate materials (electrode materials) for constructing the interdigital transducers (xe2x80x9cIDTsxe2x80x9d) and reflectors of the surface acoustic wave devices are different from each other, depending on the surface acoustic wave mode to be used and the particular application for the device. For example, as appropriate materials for the IDTs and reflectors of resonators and band-pass filters using Shear Horizontal or SH waves, metals such as Au, W, Ta, and Pt, having a specific gravity of 15 or higher are used in many cases.
Moreover, in the resonators, the band-pass filters, and such devices, in order to suppress vibration modes that deteriorate the characteristics thereof, the thicknesses of the IDT electrodes and reflectors must be within a predetermined range in some cases. If the film thicknesses of the IDT electrodes and the reflectors have predetermined values so that the vibration modes that deteriorate the characteristics can be initially suppressed, the film thicknesses will significantly depart from the values at which the surface acoustic waves can be most effectively vibrated and reflected. Thus, in some cases, a problem arises in that the necessary good characteristic cannot be obtained.
Moreover, if the film-thicknesses of the IDT electrodes and the reflectors are restricted, it will be more difficult to sufficiently reduce the resistances of the IDT electrodes, and also, problematically, the devices cannot be sufficiently adapted for operation at a high frequency.
Referring to production of multiple filters and resonators using the same wafer, it is desirable that the frequency dispersion be made as small as possible. For this purpose, advantageously, materials having a specific gravity of up to 10, such as Ni, Cu, Al, Mg, and Co, are used. However, when these materials are used, it is difficult to obtain filters and resonators having desired characteristics in practice.
Moreover, when filters and resonators including IDT electrodes and reflectors using metals with a specific gravity of 15 or higher, such as Au, W, Ta, and Pt, are produced, efforts are made to form the IDT electrode and reflector films as uniformly as possible so that the frequency dispersion of multiple surface acoustic wave devices (filters, resonators, and other such devices) produced by using the same wafer can be suppressed. However, there is a limit to the reduction in thickness of the IDT electrodes and the reflectors. Practically, the wafer is divided in individual elements, and thereafter, frequency-adjustment is carried out for each of the elements. The frequency-adjustment is made because a slight difference in film thickness between the electrode films constituting the IDT electrodes and the reflectors causes the acoustic velocity to change significantly, thereby increasing the frequency dispersion of the resonators and filters.
In order to achieve the frequency-adjustment, a method of etching the surface of an IDT electrode by using ion beams, a method of forming an insulator film on a substrate and an IDT electrode, a method of etching a substrate or an IDT electrode by RIE, are generally used. Accordingly, a problem arises in that the manufacturing cost is increased due to reduction of throughput, and moreover, the characteristics deteriorate due to damage to the electrodes and the substrates.
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave device in which frequency dispersion, influenced by dispersion in film-thickness, is minimized, while suitable excellent resonance characteristics and filter characteristics are ensured. In addition, preferred embodiments of the present invention provide a method of producing such a surface acoustic wave device.
According to a preferred embodiment of the present invention, a surface acoustic wave device includes an interdigital transducer (IDT) electrode, or an IDT electrode and a reflector, disposed on a piezoelectric substrate, each of the IDT electrode and the reflector having a multi-layer film structure including: (a) at least one high specific gravity metal component containing a layer having a film-thickness of at least about 10 nm and including as a major component a metal having a specific gravity of at least about 15; and (b) at least one low specific gravity metal component containing a layer having a film-thickness of at least about 10 nm and including as a major component a metal having a specific gravity of up to about 10 and a bulk resistivity, at 20xc2x0 C., of up to about 10xc3x9710xe2x88x928 xcexa9xc2x7m.
By forming an interdigital transducer (IDT) electrode, or the IDT electrode and a reflector on a piezoelectric substrate, each of the IDT electrode and the reflector having a multi-layer film structure including at least one layer containing, as a major component, a metal with a specific gravity of at least about 15 and a film-thickness of at least about 10 nm as a high specific gravity metal component containing layer, and at least one layer including as a major component a metal with a specific gravity of up to about 10 and a bulk resistivity, at 20xc2x0 C., of up to about 10xc3x9710xe2x88x928 xcexa9xc2x7m, and a film-thickness of at least about 10 nm as a low specific gravity metal component containing layer, excellent resonator and filter characteristics can be ensured, and also, frequency dispersion, which may be caused by dispersions in film-thickness, is minimized. Thus, it is unnecessary to perform frequency adjustment, and the manufacturing cost is greatly reduced due to the increased throughput. In addition, damage to the substrate, the IDT electrode, and the reflector is prevented, which further enhances the yield.
In particular, since the high specific gravity metal component containing layer is provided, an SH wave can be easily excited and reflected. Moreover, since the low specific gravity metal component containing layer is provided, frequency dispersion, which may occur when a plurality of filters and resonators are produced using the same wafer, is minimized. Thus, surface acoustic wave devices having excellent resonance and filter characteristics is obtained.
In addition, a vibration mode that exerts an undesirable influence over the characteristics is minimized in the film-thickness range in which high excitation and reflection efficiencies can be ensured.
Preferably, the total number of the at least one high specific gravity metal component containing layer and the at least one low specific gravity metal component containing layer which are included in the IDT electrode or the reflector is preferably in the range of from 2 to 10.
By setting the total number of the at least one high specific gravity metal component containing layer and the at least one low specific gravity metal component containing layer to be in the range of from 2 to 10, the surface acoustic wave device having excellent resonance and filter characteristics can be obtained without having to perform excessively complicated lamination processes. Thus, the present invention is highly effective and advantageous.
Also, preferably, the metal having a specific gravity of at least about 15 that constitutes the high specific gravity metal component containing layer is preferably selected from the group consisting of Au, W, Ta, Pt, and alloys containing at least one of the metals, and the metal having a specific gravity of up to about 10 and a bulk resistivity, at 20xc2x0 C., of up to about 10xc3x9710xe2x88x928 xcexa9xc2x7m of the low specific gravity metal component containing layer is preferably selected from the group consisting of Ni, Cu, Al, Mg, Co, Fe, Zn, and alloys containing at least one of the metals. Other suitable materials may also be used for the high specific gravity metal component containing layer and the low specific gravity metal component containing layer.
By using, as the metal having a specific gravity of at least about 15 that constitutes the high specific gravity metal component containing layer, a metal selected from the group consisting of Au, W, Ta, Pt, and alloys containing at least one of the metals, and as the metal having a specific gravity of up to about 10 and a bulk resistivity, at 20xc2x0 C., of up to about 10xc3x9710xe2x88x928 xcexa9xc2x7m of the low specific gravity metal component containing layer, a metal selected from the group consisting of Ni, Cu, Al, Mg, Co, Fe, Zn, and alloys containing at least one of the metals, the surface acoustic wave has a minimized frequency dispersion and excellent resonance and filter characteristics. Thus, the present invention is highly effective and advantageous.
Also, preferably, the surface acoustic wave device utilizes excitation of an SH wave.
When the present invention is embodied as a surface acoustic wave device utilizing the excitation of an SH wave, the surface acoustic wave device operates such that an SH wave is efficiently excited and reflected, and such that excellent resonance and filter characteristics and minimal frequency dispersion are achieved.
More preferably, the volume of the low specific gravity metal component containing layer that constitutes the IDT electrode or the reflector is in the range of about 25% to about 98% of the overall volume of the IDT electrode or the reflector.
When the volume of the at least one low specific gravity metal component containing layer which constitutes the IDT electrode or the reflector is in the range of about 25% to about 98% of the overall volume of the IDT electrode or the reflector, high resonator and filter characteristics is ensured, frequency dispersion, which may be caused by dispersion in film thickness, is minimized, and also, frequency adjustment for each of the surface acoustic wave devices is made unnecessary. Moreover, the manufacturing cost is greatly reduced due to the increased throughput, and the yield is greatly improved since damage to the substrate, the IDT electrode, and the reflector is prevented.
As described above, preferably, the volume of the low specific gravity metal component containing layer that constitutes the IDT electrode or the reflector is in the range of about 25% to about 98% of the overall volume of the IDT electrode or the reflector. This is because, if the volume is less than about 25% of the overall volume of the IDT electrode or the reflector, dispersion in filter characteristics, caused by dispersion in film-thickness, is increased, and if the volume exceeds about 98%, the surface acoustic wave reflectivity of the reflector becomes undesirable.
Also, preferably, the volume of the low specific gravity metal component containing layer that constitutes the IDT electrode or the reflector is in the range of about 60% to about 98% of the total volume of the IDT electrode or the reflector.
By setting the volume of the low specific gravity metal component containing layer that constitutes the IDT electrode or the optional reflector in the range of about 60% to about 98% of the overall volume of the IDT electrode or the reflector, further improved resonator and filter characteristics is ensured, and frequency dispersion, which may be caused by dispersion in film-thickness, is minimized.
Moreover, preferably, in the IDT electrode or the reflector, the low specific gravity metal component containing layer is located nearer to the piezoelectric substrate than the high specific gravity metal component containing layer.
When the low specific gravity metal component containing layer is located nearer to the piezoelectric substrate than the high specific gravity metal component containing layer in the IDT electrode or the reflector, both of the effects of the reflector on reflection of a surface acoustic wave and the effects on suppression of dispersions in frequency, are achieved.
Furthermore, preferably, the high specific gravity metal component containing layer is arranged as the outermost surface layer of the IDT electrode or the reflector.
By arranged the high specific gravity metal component containing layer as the outermost surface layer of the IDT electrode or the reflector, the frequency adjustment can be efficiently performed simply by irradiating ion beams at a low dose, if the adjustment is required. Thus, deterioration of the characteristics is minimized.
Also preferably, the low specific gravity metal component containing layer contains Ni as a major component, and the high specific gravity metal component containing layer contains Au as a major component.
By using, as the low specific gravity metal component containing layer, a layer containing Ni as a major component and, as the high specific gravity metal component containing layer, a layer containing Au as a major component, the effects of Ni on suppression of influences caused by dispersion in film thickness, and also, the effects of Au on high reflection of a surface acoustic wave can be achieved.
Furthermore, preferably, the low specific gravity metal component containing layer contains Cu as a major component.
By using as the low specific gravity metal component containing layer, a layer containing Cu as a major component, the resistivity of the film is minimized. Thus, excellent filter and resonator characteristics are obtained.
Moreover, preferably, the IDT electrode or the reflector includes the high specific metal component containing layer containing Au as a major component, the low specific gravity metal component containing layer containing Cu as a major component, and the low specific gravity metal component containing layer containing Ni as a major component, and the volume of the high specific metal component containing layer containing Au as a major component is preferably in the range of about 2% to about 30% of the overall volume of the IDT electrode or the reflector, the volume of the low specific metal component containing layer containing Cu as a major component is in the range of about 10% to about 60% of the overall volume of the IDT electrode or the reflector, and the volume of the low specific metal component containing layer containing Ni as a major component is preferably in the range of about 15% to about 78% of the overall volume of the IDT electrode or the reflector.
Since the IDT electrode or the reflector includes the high specific metal component containing layer containing Au as a major component, the low specific gravity metal component containing layer containing Cu as a major component, and the low specific gravity metal component containing layer containing Ni as a major component, and the volume of the high specific metal component containing layer containing Au as a major component is in the range of about 2% to about 30% of the overall volume of the IDT electrode or the reflector, and moreover, the volume of the low specific metal component containing layer containing Cu as a major component is in the range of about 10% to about 60% of the overall volume of the IDT electrode or the reflector, and the volume of the low specific metal component containing layer containing Ni as a major component is in the range of about 15% to about 78% of the overall volume of the IDT electrode or the reflector, undesirable waves, which may be caused when the device is used as for a resonator or a filter, are minimized, and excellent characteristics are obtained.
Also preferably, in the IDT electrode or the reflector, the low specific metal component containing layer containing Cu as a major component is preferably located between the high specific metal component containing layer including Au as a major component and the piezoelectric substrate, and the low specific metal component containing layer including Ni as a major component is preferably located between the low specific metal component containing layer including Cu as a major component and the piezoelectric substrate.
By disposing the low specific metal component containing layer including Cu as a major component between the high specific metal component containing layer including Au as a major component and the piezoelectric substrate, and by disposing the low specific metal component containing layer including Ni as a major component between the low specific metal component containing layer including Cu as a major component and the piezoelectric substrate in the IDT electrode or the reflector, both of the effects on suppression of undesired waves and the effects on reduction of a resonance resistance are achieved.
According to another preferred embodiment of the present invention, a method of manufacturing a surface acoustic wave device includes the steps of forming a plurality of IDT electrodes or a plurality of IDT electrodes and reflectors on a mother wafer substrate of a piezoelectric substrate, the piezoelectric substrate being defined by a rotated Y-cut quartz substrate having Euler""s angles expressed as (0, xcex8, xcfx86) in which xcex8 is in the approximate range of 125xc2x0 less than xcex8 less than 132xc2x0 or is equivalent to the range in such a manner that the angle xcfx86 of the surface acoustic wave propagation direction with respect to the crystal X axis is about 90xc2x0, and cutting the mother wafer at predetermined positions to divide the mother wafer into individual surface acoustic wave elements.
When the surface acoustic wave device is produced by forming the IDT electrode and the reflectors on the mother wafer, and cutting the mother wafer to divide the mother wafer into individual surface acoustic wave devices, the IDT electrodes and the reflectors have the above-described multi-layer structure, and therefore, frequency dispersion, which may be caused by dispersion in the thickness of the electrode films constituting the IDT electrode and the reflector in the individual surface acoustic wave devices, is minimized. Thus, the surface acoustic wave device of which the characteristics have lower dispersion can be efficiently produced. Moreover, frequency adjustment for each of the surface acoustic wave devices becomes unnecessary. Thus, the manufacturing cost is greatly reduced, throughput is greatly increased, and a high yield is obtained while damage to the substrate, the IDT electrode, and the reflector is prevented.
Preferably, the surface acoustic wave device of preferred embodiments of the present invention is produced by the above method, wherein the device is formed by cutting the mother wafer having the plurality of IDT electrodes, or the plurality of IDT electrodes and the plurality of reflectors, so as to divide the mother wafer into individual surface acoustic wave elements.
Since the surface acoustic wave device of various preferred embodiments of the present invention is produced by cutting the mother wafer having the plurality of IDT electrodes or the plurality of IDT electrodes and the plurality of reflectors, and the IDT electrodes and the reflectors have the multi-layer structure, frequency dispersion is minimized and frequency adjustment for each of the surface acoustic wave devices is not needed. Accordingly, a surface acoustic wave device having excellent resonator and filter characteristics and minimal dispersion in characteristics is provided at low cost.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments with reference to the attached drawings.